Electrowinning Aluminum in the Hall-Heroult Cell
Since the patenting of the Hall-Heroult cell ("HHC") in 1886 for electrowinning liquid aluminum (Al) at about 960.degree. C. , the basic features have remained the same, although obviously significant optimization of the process variables has occurred. (1) Even today, liquid Al is deposited into a carbon (cathode) hearth, having sidewalls protected by frozen crust, by electrochemically reducing alumina (Al.sub.2 O.sub.3) dissolved in a fused fluoride electrolyte.
The principal component of the fused electrolyte is cryolite (Na.sub.3 AlF.sub.6), although the NaF/AlF.sub.3 bath ratio has been optimized and other bath additions (e.g., LiF, CaF.sub.2, MgF.sub.2) have been made. The electrolyte serves as the solvent for alumina derived from bauxite ore, typically purified by the Bayer digestion process. Most important as backdrop for the process improvement disclosed herein, the modern version of the HHC runs the anodic oxidation reaction at an expensive prebaked and refined carbon anode, resulting in the oxidation and consumption of the carbon to release CO.sub.2 product gas. The present invention will also apply to the replacement of the older, but still currently used process, involving a Soderberg carbon anode.
As is well understood by the industry worldwide, there are many problems associated with the use of the consumable carbon anode. First, the stoichiometric consumption of the carbon anode, according to the reaction EQU 1/2Al.sub.2 O.sub.3 +3/4C=Al+3/4CO.sub.2 (1)
represents a significant cost for the carbon, amounting to about 14.4% of the cost of producing primary Al. (2) However, the formation of the CO.sub.2 gaseous product from carbon oxidation offers the advantage that the thermodynamic (open-circuit) voltage for Eq. (1) is held down to 1.20 volts; but then the anodic oxidation reaction has a significant overvoltage of about 0.5 volts, while another 0.35 volts are required to pass the high current through the anode. Furthermore, the uneven oxidation of carbon results in a rounding of the originally flat anode geometry which necessitates a significant anode-to-cathode spacing of about 5.0 cm (to avoid shorting) and thereby a significant IR drop (about 1.45 volts) through the electrolyte, requiring periodic anode adjustment. The evolution of CO.sub.2 bubbles at the carbon anode also introduces an additional polarization of about 0.30 volts, while some back reaction between the CO.sub.2 product and the reduced aluminum lowers the current efficiency for the particular materials and process variables used. Since the electrical cost of the electrolysis process is directly proportional to both the applied cell voltage and the current efficiency, the elimination, or minimization, of certain of these contributions to the cell voltage could lead to a significant reduction in the cost of producing primary aluminum, as will be demonstrated later.
As an additional important factor opposing the continued use of the carbon anode, the release of the greenhouse gas CO.sub.2 by the process is meeting increasing environmental objection. While the stoichiometric requirement for anode carbon according to Eq. (1) is 0.33 #C/#Al, in fact, direct oxidation and other losses result in the consumption of about 0.45 #C/#Al, amounting to the release of 1.65 #CO.sub.2 /#Al.(2-5) Further, from an environmental standpoint, in addition to CO.sub.2, the fabrication and oxidation of carbon anodes also evolve objectionable HF, CO, perfluocarbon volatiles, and other volatile organic compounds (VOC). (5) The equipment and associated maintenance and labor to reduce these emissions inherent to the use of the carbon anode represent a significant cost and problem for the primary aluminum producers.
Through Faraday's Law, the rate of Al production is established (for 100% current efficiency) by the cell current. But a significant cost of electrical energy is required for electrowinning Al. The current US composite baseline energy use is estimated to be 15.2 kWh/kg Al. About 22.8% of the total cost is proportional to the impressed cell voltage which constitutes the summation of the thermodynamic (open-circuit) voltage, the anodic and cathodic overpotentials, bubble effects, the IR drop in the fused salt electrolyte, plus voltage drops in the electrodes and collector bars external to the cell, etc. (4)
The modern HHC operates today at about 4.4 volts with a current efficiency of about 95%. The heat balance for the cell is maintained by providing sufficient insulation so that the I.sup.2 R heat generated in the electrolyte keeps the cell at the operating temperature of about 960.degree. C.
The maximum permissible anodic current density, which limits the rate of Al production, is set by the occurrence of the "Anode Effect." When the local concentration of alumina dissolved in the electrolyte becomes too low, CO.sub.2 evolution at the anode is interrupted, a passivating/insulating film of very environmentally objectionable fluorocarbon (CF.sub.4 and C.sub.2 F.sub.6) species cover the anode, and the cell must receive immediate attention (i.e., in order to force alumina replenishment to the electrolyte) to resume production. Of course, the Anode Effect cannot be blamed solely on the nature of the anodic oxidation reaction on carbon, but also on the difficulty to dissolve sufficient alumina rapidly enough to support the anode reaction.
The Non-Consumable Anode
Because of the many problems inherent to the carbon anode (high anodic overvoltage, production-limiting current density, high IR drop, objectionable environmental impact, and high cost), researchers dating back to Charles Martin Hall in 1880's have tried to develop a non-consumable (inert) anode ("NCA") comprising an alloy, oxide or a metal/oxide cermet with an oxidized surface where pure oxygen could be evolved as the anodic oxidation reaction to accomplish the following total cell reaction: EQU 1/2Al.sub.2 O.sub.3 =Al+3/4O.sub.2 (2)
While the thermodynamic (open-circuit) voltage of 2.20 volts for reaction (2) using an NCA is higher than the 1.20 volts for reaction (1), the anodic overvoltage amounts to only about 0.10 volts, i.e. much less than the 0.50 volts for the carbon anode. Because of the dimensional stability of an NCA, the anode-to cathode spacing (ACS) could be reduced from about 5.0 cm for carbon to about 3.5 cm for the NCA, with a corresponding decrease in IR drop, and other smaller reductions in required cell voltage would be realized. On the balance, the electrical cost of Al production using the NCA, with no other improvements, should be somewhat less that for a consumable carbon anode. (6) Besides, the significant cost for the carbon anode would be saved and the environmentally objectionable CO.sub.2 and other product gases would be replaced by O.sub.2. Despite these potential advantages and significant research and development efforts, an acceptable NCA has not yet been inserted into the HHC. The status and development of the NCA were reviewed recently by Thornstad and Olsen. (8) The leading NCA candidate is comprised of the oxides of Fe and Ni, with some copper metal to increase the conductivity. Fine oxygen bubbles are formed at the oxide surface of this "inert" anode contacting the cryolite bath. But the solubilities of NiO and NiFe.sub.2 O.sub.4 in the electrolyte at normal HHC temperatures are too high, so that unacceptable levels of Ni and Fe impurities are deposited into the Al. However, modifications in the electrolyte composition, e.g. additions of AlF.sub.3 and LiF, and the use of a controlled gas evolution to maintain cryolite/alumina slurry, would permit a significantly lower temperature (as low as 685.degree. C. ) for operation of the electrolysis cell and therewith lower solubilities for the offending Ni and Fe solutes. (5, 9) On the whole, an effective NCA would displace the carbon anodes, and existing Al electrowinning cells would be retrofitted with the new NCA. (6)
In a parallel effort to lower the voltage required for the HHC, research into the identification and development of an "inert cathode" is also well established. The high magnetic fields developed during electrolysis induce convection currents in the Al pad, producing an uneven, wavy surface facing the anode. To avoid shorting, the anode-cathode distance ("ACD") must be increased, corresponding to an increased IR drop in the cell, compared to that ACD permitted if the cathode surface were fixed and stable. Inert cathodes are known which contact, but project above the Al pad into the electrolyte, so that the liquid Al deposited at the inert cathode drains into the Al pad below. The leading material candidates involve a TiB.sub.2 plus graphite composite, or a TiB.sub.2 coating of graphite. However, because of fabrication difficulties and inherently inadequate mechanical properties, the inert cathode has also not been substituted into industrial practice.
The Solid Electrolyte Non-Consumable Anode
In U.S. Pat. Nos. 3,562,135 and 3,692,645, Marincek proposed to electrolyze alumina dissolved in the fused cryolite-base electrolyte by separating the anode from the salt by a thin dense layer of an oxygen-ion conducting solid electrolyte. The zirconia layer (stabilized by calcium oxide or other oxides) would support the electrically driven migration of oxide ions from the melt being electrolyzed, but would be non-permeable to and resistant to the melt at the temperature of electrolysis. At a catalytic, electronically conducting, porous anode inside the hollow structure, pure oxygen gas would be evolved from the electrochemical oxidation of the oxide ions migrating in the solid electrolyte. In principle, the pure oxygen evolved could be collected and sold or used elsewhere in the plant.
These patents by Marincek have not led to a commercial acceptance of this concept, although the scheme achieves essentially the net result (evolution of pure oxygen) as the non-consumable anode under development today. In fact, at the 960.degree. C. normal operating temperature of the HHC, the solubility of zirconia in the cryolite-base melt is quite high about 0.4 and 0.8 wt % Zr for melts containing 6.5 and 2.5 wt % dissolved Al.sub.2 O.sub.3, respectively.(7) But the solubility drops significantly with reduction in temperature, and new technology is available to run a HHC at significantly lower temperature. Furthermore, zirconia is one of the few oxides that does not react with AIF.sub.3 to form ZrF.sub.4 and Al.sub.2 O.sub.3, and has a decomposition potential approximately equal to Al.sub.2 O.sub.3.